As a consequence of higher demand for data traffic in cellular systems, there is an increasing trend towards a denser deployment of base stations or access points to increase the data rate of users. As a result, new deployment models, such as femtocells have been considered by many mobile operators as critical factors in the growth of the cellular market.
However, dense deployment of base stations or access points comes with their own set of problems, of which, the most significant is inter-cell interference. Interference that comes with dense home deployments may have to be solved before such deployment scenarios can be seriously considered.
For femtocellular deployments, there are two main interference scenarios which operators are especially concerned with. The first, is the downlink interference a femtocell access point (FAP) can cause to user equipments (UEs) attached to another macrocell base station (MBS) (from now on denoted as macrocell UE or MUE) due to the utilization of the same frequency spectrum as shown in FIG. 1. The second is the uplink interference caused by MUEs to FAPs as shown in FIG. 2.
In interference scenarios, there normally exist one or more interference source(s) or aggressor(s) and, there are two ways of mitigating interference. One is to operate on the interference source or aggressor and modify its transmission characteristics such that the interference felt at the victim is reduced. The other alternative is to change the reception behaviour of the victim such that it is less susceptible to interference from the aggressor.
One method of reducing the interference experienced at the receiver is to carefully plan the frequency allocation in the neighbouring cells so as to minimise interference at the receiver. However, there are some drawbacks of this method. Firstly, it requires extensive exchange of data between the base stations or feedback of a large amount of information to a central server which can coordinate this allocation. Additionally, the frequency reuse factor is worse, hence, the spectrum is used less efficiently. Moreover, this technique is not feasible for femtocell base stations, which do not have the capability to communicate between neighbouring femtocell base stations. Additionally, feedback of large amount of information for such purposes will cripple the home broadband which supports the femtocells.
Coordinated transmission is an alternative to frequency allocation. This method avoids interference by cancelling the interference prior to the transmission. However, such a method requires exchange of a large amount of data between base stations. Consequently, such methods are not feasible for implementation in femtocells.
The problem with using resource allocation or coordinated transmission to resolve interference situation is the amount of prior knowledge they require to complete the task. Both methods require large amounts of knowledge, such as channel state information at multiple locations. For example, to feedback a single channel coefficient to 10 different interference nodes quantised at 8 bits would require 80 bits, and for 20 nodes would require 160 bits. The number of bits required for feedback would scale linearly with the number of interference nodes.